[jdk17u-dev] RFR: 8291775: C2: assert(r != __null && r->is_Region()) failed: this phi must have a region

[Goetz Lindenmaier]

I backport this for parity with 17.0.6-oracle.

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Commit messages:
 - Backport 32d675ca607d341ca3428efc32e212701775e3c6

Changes: https://git.openjdk.org/jdk17u-dev/pull/760/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=760&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8291775
  Stats: 8 lines in 2 files changed: 5 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/760.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev pull/760/head:pull/760

PR: https://git.openjdk.org/jdk17u-dev/pull/760